The present invention relates to a tension member intended for use primarily in connection with tension legs for a tension leg platform, but other applications are also relevant, such as in stays or wires for bridges (for example, suspension bridges or inclined strut bridges), anchoring of underwater tunnels, or other uses where there is a need for a light and strong wire or stay. The invention is therefore not limited to the utilization described in the following detailed description.
Tension leg platforms are widely used in drilling and production in oil fields where for various reasons it is not possible or economically justifiable to install a permanent platform, and where it would not be practical to use a floating platform anchored by means of anchors and anchor chains.
The tension leg platforms are in principle floating platforms where, however, instead of a slack anchoring with the aid of anchors and anchor chains, there are tension legs extending from the platform approximately vertically down to an anchor on the seabed. The tension legs are placed under a substantial degree of tension so that, to the extent possible, the platform will be maintained in the same position relative to the seabed. The platform's stable position is a great advantage in both drilling and production. However, this places high demands on the tension legs being used and on their attachment to the platform and their anchoring on the seabed.
The tension legs most widely used today consist of steel tubing in sections. The sections may have unequal lengths, have unequal diameters, and exhibit various wall thicknesses, depending on the size of the platform and the depth of the water. The legs are always constructed as tubes having an air-filled cavity, so that the weight of the leg in the water is greatly reduced. This places a lighter load on the platform. The dimensioning of the leg in relation to external water pressure is therefore a design criterion. These steel legs function well at moderate depths, i.e., depths of a few hundred meters. However, oil and gas production now takes place at increasingly greater depths, possibly up to 2000 meters. Under such conditions there are great demands placed on the strength of the tension legs, and a tension leg of steel would not be usable. The thickness of the wall would then, out of consideration for the increased water pressure, have to be very great, and the pipes would thereby become extremely heavy. For transport reasons they would also have to consist of a great many sections that would need to be joined together during installation. The tension legs would thereby acquire a considerable number of joints, which would also contribute to the substantial weight increase. To counteract the increase in weight, it could be advisable to equip the legs with a large number of buoyancy members. All this would result in an extremely expensive and heavy installation.
Carbon fibers, with their low weight and high tensile strength, have already been put to use in various areas in connection with oil and gas extraction, for example, as hoisting cables at great depths, where the weight of a hoisting cable made of steel would create problems.
It is an aim according to the present invention to exploit the advantageous properties of the carbon fibers, particularly their high strength when subjected to tensile stresses, by utilizing them also in tension legs. However, the carbon fibers do also have one significant negative property: they have very little rupture strength in the face of shearing stresses. In the designing of a tension leg consisting of carbon fibers, this factor would have to be taken into consideration.
From the present applicant's own Norwegian patent 304839 (corresponding to WO 98/39513) there is known a tension member incorporating ideas from the applicant's own pipe bundle cable (umbilical) as described in NO 155826. Here, a plurality of smaller pipelines are laid in a bundle in a manner that allows them axial movement in relation to each other. The cable is not suitable, however, for taking up a high degree of tension.
NO 174940 describes a method and a machine for combining a plurality of elongate pipelines or cables into a cable string (umbilical). This cable string comprises a center tube. Nor in this case is the cable string suitable for absorbing substantial tension.
EP 685 592 describes a method for separating the individual strands in a steel wire in order to prevent wear and to increase the cross section. The plastic elements between the strands will be pressed together when a load is placed on the cable, and will thereby prevent contact between the strands. The strands are not capable of free axial movement in relation to each other due to this compression, or clamping effect.
FR 2078622 also describes a steel wire into which is laid a filler substance to separate the individual wires. Free axial movement of the strands is hampered as direct contact occurs between them.
U.S. Pat. No. 3,088,269 describes a method for manufacturing a steel wire having a smooth surface for use in aerial cableways and the like. The filler elements are inserted between the strands in order to secure them and to hold them apart from one another. There is no possibility of free movement between the strands here, either, since the intention is, quite to the contrary, to achieve a clamping effect between the strands and the elements.
The solution according to the aforementioned Norwegian patent 304839 aimed to provide a tension leg made preferably of carbon fiber, which would be usable for tension leg platforms at great depths, where the carbon fibers were protected against shearing stresses by means of pressure-proof spacing elements having recesses into which the strands were laid individually in a manner permitting them to move in the longitudinal direction unobstructed by one another or by the spacing elements.
Although this solution functions very well, however, the construction is complicated and the manufacture is difficult. The spacing elements also contribute to an enlarged diameter, which is a disadvantage when the tension member is to be coiled up, and to increased weight. Moreover, the spacing elements make the tension member more expensive.